1. Field of the Invention
The present invention relates generally to shaft seals and, more particularly, is concerned with a double dam seal for a reactor coolant pump having a self-contained injection pump mechanism.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F and 2250 psi. The pump basically includes three general sections from bottom to top--hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem sealing assemblies--lower primary, middle secondary and upper tertiary sealing assemblies. The sealing assemblies are located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition. Representative examples of pump shaft sealing assemblies known in the prior art are the ones disclosed in MacCrum U.S. Pat. No. (3,522,948), Singleton U.S. Pat. No. (3,529,838), Villasor U.S. Pat. No.(3,632,117), Andrews et al U.S. Pat. No. (3,720,222) and Boes U.S. Pat. No. (4,275,891) and in the first three patent applications cross-referenced above, all of which are assigned to the same assignee as the present invention.
The lower primary sealing assembly (or No. 1 seal), the main seal of the pump, is a controlled-leakage film-riding face seal. Its primary components are a runner, which rotates with the shaft, and a non-rotating seal ring, which is attached to the lower seal housing. The No. 1 seal causes a pressure drop of coolant water from about 2250 psi to 30 psi across its face and allows a flow rate of about 2-3 gpm therethrough. The lowpressure coolant water leaking through the No. 1 seal flows up the shaft annulus to the region of the middle secondary sealing assembly.
The middle secondary sealing assembly (or No. 2 seal) is a rubbing face-type seal. Its primary components are a rotating runner and a non-rotating ring. During normal operation, the ring and runner provide a rubbing seal. If the No. 1 seal fails, however, the distribution of pressure on the No. 2 seal runner causes it to act as a spring and deflect in such a way as to provide a film-riding face seal. Most of the coolant water from the No. 1 seal is diverted to the No. 1 seal leakoff. However, a portion of the water passes through the No. 2 seal, leaking at a flow rate of about 2 gph at a pressure drop of from 30 psi to 3-7 psi. The still lower pressure coolant water leaking through the No. 2 seal flows further up the shaft annulus to the region of the upper tertiary sealing assembly.
The upper tertiary sealing assembly (or No. 3 seal) is also a rubbing face-type seal, its primary components being a rotating runner and a non-rotating ring. Most of the flow leaking from the No. 2 seal is diverted by the No. 3 seal out through the No. 2 seal leakoff.
The No. 3 seal typically has one of two forms: either its rubbing face-type seal has a double face or dam with two concentric sealing faces, or it has a single face or dam. In the case of the double dam seal, clean water at a slightly elevated pressure (8-10 psi) over that at the No. 2 seal leakoff (7 psi) is injected as a buffer fluid into the annulus between the two faces or dams such that (1) a portion of this injected flow goes back outward past the outer or upstream one of the seal faces or dams into the housing cavity between the No. 2 and No. 3 seals and then out the No. 2 seal leakoff, and (2) another portion of this injected flow goes in the opposite direction inward past the inner or downstream one of the seal faces or dams and ultimately to a No. 3 seal leakoff to the containment atmosphere. Thus, it is seen that this injection of clean or pure water prevents radioactive gases in the reactor coolant water from passing between the double dams and into the containment atmosphere. On the other hand, in the case of the single dam seal, no injection of clean water is employed. Instead, a portion of the reactor coolant water leaks past the single dam seal and therefrom into the containment atmosphere.
The double dam design permits design control of the hydraulic forces acting on the seal ring by adjustment of the geometry of the noses and the pressure of the injected buffer fluid. The closing forces or nose loads can thereby be tailored to control leakage and to reduce heat generated by the friction in the running seal. In the single nose design there is less latitude in control of the nose load, the body weight of the ring assembly playing the major role in determining the nose load. In some applications of single nose designs the pressure of the sealed fluid may be too low to support adequate leakage to cool the seal. The double dam seal with its buffer fluid injected at a higher pressure assures adequate leakage and cooling.
The double dam-type seal is preferred for its greater flexibility in design as well as for preventing contaminated reactor coolant water from flowing through the No. 3 seal to the containment environment. However, heretofore, the injection water has had to be supplied by a system of external pipes and tubes and internal flow passages. In reactors not originally equipped with such an external system, it is not practical economically to now attempt to retrofit the nuclear power plant so that clean or filtered water can be supplied for injection at the No. 3 seal.
Consequently, a need exists for an alternative means to pressurize the annulus between the noses of the double dam seal which does not require an external supply system.